As is known in the art, a light source or lamp generally refers to an electrically powered element which produces light having a predetermined color such is a white or a near white. Light sources may be provided, for example, as incandescent light sources, fluorescent light sources and high-intensity discharge (HID) light sources such as mercury vapor, metal halide, high-pressure sodium and low-pressure sodium light sources.
As is also known, fluorescent and HID light sources can be driven by a ballast. A ballast is a device which by means of inductance, capacitance or resistance, singly or in combination, limits a current provided to a light source such as a fluorescent or a high intensity discharge light source, for example. The ballast provides an amount of current required for proper lamp operation. Also, in some applications, the ballast may provide a required starting voltage and current. In the case of so-called rapid start lamps, the ballast heats a cathode of the lamp prior to providing a strike voltage to the lamp.
As is also known, a relatively common ballast is a so-called magnetic or inductive ballast. A magnetic ballast refers to any ballast which includes a magnetic element such as a laminated, iron core or an inductor. Magnetic ballasts are typically reliable and relatively inexpensive and drive lamps coupled thereto with a signal having a relatively low frequency.
FIG. 1 shows an exemplary prior art magnetic ballast 10 for energizing a lamp 12. The ballast 10 includes an inductive element or choke L and a capacitive element C which is coupled across first and second input terminals 14a,b of the ballast. The capacitive element C provides power factor correction for an AC input signal. In an exemplary embodiment, the choke has an impedance of about 1.5 Henrys and the capacitor C has a capacitance of about 3 microFarads.
The input terminals 14a,b are adapted for receiving the AC input signal, such as a 230 volt, 50 Hertz signal. The first input terminal 14a can be coupled to a so-called Phase (P) signal and the second input terminal 14b can be coupled to a so-called Neutral (N) signal. The lamp 12 includes first and second lamp filaments FL1,FL2 with a starter circuit 16 coupled in parallel with the lamp filaments. Upon initial application of the AC input signal, the starter circuit 16(, provides a short circuit so that current flows through the starter circuit thereby heating the lamp filaments FL1,FL2. After a time, the starter circuit 16 provides an open circuit as current flow through the lamp 12 is initiated. A voltage level of about 230 Volts is sufficient to strike the lamp 12 and cause current to flow between the filaments FL1,F12.
While such a circuit configuration may provide an adequate power factor, it is relatively inefficient and generates significant heat that must be dissipated. In addition, the circuit requires a starter circuit to initiate current flow through the lamp. Furthermore, the circuit is not readily adapted for providing a lamp dimming feature.
It would, therefore, be desirable to provide a ballast circuit that is efficient and allows the light intensity to be readily modified, i.e., dimming.